The congresswoman from Irvine, California, announced her decision in a video statement. About 60 other lawmakers support opening an inquiry, a far cry from a majority in the 435-seat House.
Porter said she believes Congress must investigate after special counsel Robert Mueller said he couldn't exonerate Trump of obstruction of justice in a probe into Russian interference in the 2016 election and didn't have the option to indict a sitting president.
"I cannot with a clean conscience ignore my duty to defend the Constitution. I can't claim to be committed to rooting out corruption and putting people over politics and then not apply those same principles and standards in all of the work I do," Porter said.
She called out the administration for ignoring subpoenas and directing officials to disregard summons to testify before Congress.
"The administration has refused to respect the rule of law," Porter said. "The question is not whether a crisis is in our midst but rather whether we choose to fight against it."
Porter, a consumer protection lawyer and law professor at the University of California, Irvine, was elected last year in one of several contested races that put all the traditionally Republican districts in Orange County under Democratic control amid Trump's low approval ratings.
But Republicans still outnumber Democrats in her district covering a swath of eastern Orange County and hold a slight edge in the county as a whole. Porter already has a number of Republican challengers seeking her seat in 2020.
Fred Whitaker, chairman of the Orange County Republican Party, called Porter "too far left" for her district and "out of touch."
"She doesn't have the support of her district. She doesn't even have the support of Speaker Pelosi," Whitaker said.
"Most people think we should accept the Mueller report and move forward," he said.
In Porter's district, about 30.8 percent of registered voters are Democrats and 35.7 percent are Republicans, according to the county registrar.
In a May survey by the Public Policy Institute of California, about 38 percent of respondents in Orange and San Diego counties said they approved of the way Trump was handling his job as president.
At a town hall in Orange County earlier this month, Porter said she didn't seek election to impeach Trump but she felt the refusal to comply with subpoenas was a turning point and wouldn't refrain from doing so if the time came.
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